In a projection device of a projection type television receiver, there is commonly provided transparent liquid or viscous body having a light refractive index close to that of the glass between the lens and the CRT tube. This is provided so as to prevent the decrease in contrast caused by the video image light emitted from the CRT fluoresent screen which is reflected at the boundary between the air existing between the CRT and the lens, and the glass face surface of the CRT. This produces an advantageous increase in the luminance of the video image by the extent that the reflection is decreased.
At first, a prior art projection device is described with reference to FIG. 1:
A lens unit 1 is fixed to the bracket 4 which supports the whole device by a screw inserted through an opening of a fitting flange 2. A CRT 5 is positioned at the side of the lens 3 of the lens unit 1 via an O-ring 7. The CRT 5 is fixed to the bracket 4 by a screw inserted through an opening of the ear section 6. Transparent silicon gel 8, for example, provided at the inside of the O-ring 7.
The silicon gel 8 expands or contracts to change the volume thereof in accordance with its temperature. Accordingly, the lens 3 in contact with the silicon gel 8 in the lens unit 1 is loosely mounted so as to enable the movement of the lens in accordance with the volume variation of the silicon gel 8.
The prior art projection device under such a construction has the following drawbacks:
1. The relative positions of the lenses in the lens unit 1 change in accordance with the volume variation of the silicon gel 8, thereby leading to a deviation of the function of the lens unit 1 from an optimum point. This problem is likely to arise because the CRT 5 is usually heated when functioning.
2. It is difficult to fill the O-ring 7 completely with the silicon gel 8.
3. Although the setting of the distance between the lens 3 and the CRT 5 and the relative angle between the confronting surfaces require a high accuracy, it is generally difficult to ensure the positional accuracy of the fitting ear section 6 of the CRT 5. Accordingly, it is necessary to conduct a dimension inspection while fitting the fitting ear section 6 to the bracket 4, and to provide an adjustment such as inserting a washer between the bracket 4 and the fitting ear section 6.
As evident from the foregoing, there are problems in both function and construction aspects.